The present invention relates to a technique for detecting electrical leakage in an alternating current line.
An electrical leakage breaker is used to protect an alternating-current (AC) line 110 (AC: Alternating Current) and peripheral circuits from electrical leakage. FIG. 1 is a circuit diagram of a conventional electrical leakage breaker 100R. The electrical leakage breaker 100R includes a switch (also referred to as a contact or a breaker) 102, a zero-phase-sequence current transformer (ZCT) 104, and an electrical leakage detection circuit 200R. The switch 102 is provided on an AC line 110 and is normally in a contact closed state (ON). When an electrical leakage is detected, the switch 102 is changed to a contact open state and breaks the AC line 110. The AC line 110 may be multi-phase or single-phase.
When an electrical leakage occurs due to a ground fault or other issue in the AC line 110, the zero-phase-sequence current transformer 104 generates an alternating detected current S1 on a secondary side of the ZCT 104. In a normal state, the detected current S1 is close to 0. The detected current S1 is converted into a detected voltage S2 by a resistor R1. FIGS. 2(A) and 2(B) are waveform diagrams of the detected current S1 during normal time and electrical leakage. FIG. 2(A) shows the waveform during normal time and FIG. 2(B) shows the waveform while electrical leakage occurs.
As shown in FIG. 1, the electrical leakage detection circuit 200R determines the presence or absence of electrical leakage based on the detected voltage S2 input to its input terminal (IN). When electrical leakage is detected, the electrical leakage detection circuit 200R drives a thyristor 106 connected to its output terminal (OUT) to turn off the switch 102.
The electrical leakage detection circuit 200 includes an amplifier 202, a comparator 204, a judgment circuit 210, and an output stage 206. The amplifier 202 amplifies the detected voltage S2 as necessary. The comparator 204 compares an output signal S3 of the amplifier 202 with a predetermined threshold voltage VTH1, and generates a comparison signal S4 indicating a comparison result. Based on the comparison signal S4, the judgment circuit 210 judges the presence or absence of electrical leakage and generates a judgment signal S5 indicating a judgment result. Since the amplifier 202 is an inverting amplifier in this example, when an electrical leakage occurs, the detected signal S3 exceeds the threshold voltage VTH1 (i.e., the detected signal S2 is less than the negative threshold voltage). The judgment signal S5 is asserted (for example, set to high level) when electrical leakage is judged to exist, and the judgment signal is negated (set to low level) in a normal state. When the judgment signal S5 is asserted, an output stage 206 latches the state and asserts (for example, high level) a drive signal S6 to fix the thyristor 106 in a driving state. Thus, once electrical leakage is detected, the switch 102 is kept in an OFF state until the output stage 206 is reset.
Noise enters the electrical leakage breaker 100R and the electrical leakage detection circuit 200R from various paths. For example, noise can be mixed as electromagnetic waves, lightning surge, current fluctuations of load equipment, and others. If electrical leakage is erroneously detected due to noise, the switch 102 is turned off and the downstream circuit becomes inoperative. Therefore, countermeasures against noise are necessary for the electrical leakage detection circuit 200R.